[Mysql] index back to the table covering index index pushdown

Index Type

Index primary key index and a sub-type of non-primary key index. (Be sure to keep in mind is how to store data)

• The leaf nodes of the primary key index is stored entire row of data. In InnoDB, the primary key index is also called clustering index (clustered index).
• Non-primary key index leaf node content is the value of the primary key. In InnoDB, the non-primary key index is also called the secondary index (secondary index).

Clustered index

Clustered default primary key index, if the table does not define the primary key, InnoDB selects a unique index instead of a non-empty. If there is no such index, InnoDB implicitly defined as a primary key clustered index.
B + TREE reasons, it is best to use consecutive integers field, to find a better access range determined

Back to the table

The difference between the query index structure described above, the primary key index and the ordinary index

• If the statement is select * from T where ID = 500, i.e., the primary key query, the tree only needs to search for a B + tree ID;
• If the statement is select * from T where k = 5 , that is, the general index query, you'll need to search the index k tree, get a value of 500 ID, to the ID search index tree once. This process is called back to the table .

Covering index

If the statement is executed select ID from T where k between 3 and 5, then only need to check the ID value, and the value of the ID has the index k in the tree, so you can direct the query results, you do not need back to the table. In other words, this query inside, the index k has been "covered" the needs of our inquiry, we called a covering index.

Index pushdown

Index pushdown is MySQL5.6 version of the launch of the optimization methods
is enabled by default, can be turned off by the following command

SET optimizer_switch = 'index_condition_pushdown=off';

If there about SQL, unreasonable implementation process, the following figure shows

index(name,age)
mysql> select * from tuser where name like '张%' and age=10 and ismale=1;

You can see, no index pushdown is required every time the query back to the table, and push down the previous filter is a good result set back to the table to get the data select *

No index pushed down

the index pushdown